In aerospace systems, such as engine exhaust ducts, nose cones, firewalls, and reentry shield surfaces, surfaces may be exposed to high temperatures or large temperature gradients and must, accordingly, be insulated. Each application has unique problems which have rendered it difficult to provide an adequate thermal insulation that can be tailored for optimum performance in each application.
Recently, low-density ceramic fibers have been used for insulating aerospace surfaces. For example, the space shuttle's exterior surface is insulated with a plurality of ceramic tiles that are arranged in a closely spaced, ordered array. To provide the required fit, each tile is cut precisely from a fused ceramic blank. To form the blanks, silica fibers and other ceramic components were initially mixed into a slurry and cast into blocks. After drying, the blocks were sintered to form strong ceramic bonds between the overlapping fibers. The blocks were cut into smaller blanks that were subsequently milled into the final tiles. Each tile was bonded to an isolation pad with a high-temperature adhesive. The pad was, then, bonded to the underlying metallic substructure of the shuttle.
During takeoff and reentry, a diff surface temperature distribution exists over the surface of the space shuttle. The fused ceramic tiles are vulnerable to shear forces caused by the differential surface temperature ion. To prevent breakage, each tile must be small (generally less than ten inches on a side) thereby creating enormous fabrication and assembly costs.
Glass coatings have been developed to improve thermal shock resistance for ceramics. U.S. Pat. No. 4,093,771 to Goldstein et al. discloses a borosilicate glass coating that is used on the surface of reusable silica insulation. U.S. Pat. No. 4,381,333 to Stewart et al. discloses a two-layer glass coating for silica insulation. The base layer has a high emittance and is preferably formed by combining a reactive borosilicate glass with an emittance agent, such as silicon tetraboride, silicon hexaboride, boron, or silicon carbide. The outer layer is formed from discrete, sintered glass particles to provide a high scattering coefficient. Preferably, fused silica or a reactive borosilicate for the outer layer. In either the Goldstein or Stewart patent, the coating is sprayed onto the underlying fiber ion before firing to form a glass.
Insulation may be formed with an unsolidified silica glass felt sandwiched between silica glass fiber. The three layers are stitched together with silica glass thread (or another suitable refractory thread) and are bonded with adhesive to the surface to be protected. Similarly, a layering effect may be achieved by superposing a stitched blanket of silica and aluminoborosilicate fibers (commercially available under the trademark NEXTEL from 3M Company) over a separate, stitched blanket of silica fibers. By staggering the blankets and using suitable emittance coatings on the outer surfaces of the blankets, control of the insulative characteristics can be achieved, thereby countering the temperature distribution on and gradient through the insulation.
Lightweight fibrous insulation that permits a wide range of design choices in terms of insulative characteristics, strength, and durability is still needed.